Low Mass Low Density (LMLD) planets are an exciting emerging class of exoplanets and we expect a plethora of new data in the near future. How these worlds formed and what they are made of are hotly debated issues. Suggestions for the nature of e.g. GJ1214b include (i) a hot Super-Earth with a remnant hydrogen atmosphere (possibly including an aerosol layer) (ii) a hot steam atmosphere, or (iii) a mini-Neptune with an accreted hydrogen-helium atmosphere. The purpose of this session is to bring together expertise from the observational (e.g. transmission spectra) and modeling (population synthesis, aerosol formation, photochemical-climate, atmospheric parameter retrieval, interior modeling and EOS, etc.) communities related to LMLD science. Papers are welcome e.g. on what observational data can we gather now and (based on near-term missions) in the near future in order to address the composition question (e.g. how well does the Rayleigh slope have to be measured in order to constrain potential haze layers, what resolution is required to distinguish solar composition from higher metallicities such as steam atmospheres etc.). Key modeling issues include addressing the assumptions of the population synthesis models (e.g. effect of model results to input conditions such as planetesimal size, influence of escape processes, etc.), whereas for the photochemical and aerosol modeling, key issues include better constraining chemical input (e.g. absorption coefficients and rate constants) in hot, steamy and/or solar-like atmospheres.